1. Field of the Invention
The present invention relates to a magneto-impedance element comprising a glassy alloy which is composed of at least one base metal selected from the group consisting of Fe, Co and Ni; at least one additional metal selected from the group consisting of Zr, Nb, Ta, Hf, Mo, Ti and V; and B.
The present invention also relates to a magnetic head having the magneto-impedance element.
The present invention further relates to a thin film magnetic head comprising an upper core and a lower core which have the magneto-impedance element.
The present invention also relates to an azimuth sensor having the magneto-impedance element.
The present invention further relates to an atutocanceler having a magnetic sensor composed of the magneto-impedance element.
2. Description of the Related Art
With rapid progress in development of information devices, gauging devices, and control devices, magneto-detective elements, which have a smaller size, higher sensitivity and more rapid response than conventional magnetic-flux type elements, have been required. Elements having a magneto-impedance effect, i.e., magneto-impedance elements (hereinafter referred to as MI elements) have attracted attention.
The magneto-impedance effect indicates a phenomenon causing a change in impedance in, for example, a closed circuit as shown in FIG. 5. When an alternating current Iac having a MHz band is applied to a wire or ribbon magnetic material Mi through an electrical power source Eac while an external magnetic field Hex of several gausses is applied in the longitudinal direction of the magnetic material Mi, a voltage Emi by an impedance inherent in the magnetic material occurs between two ends of the magnetic material Mi, and its amplitude varies within a range of several tens percent in response to the intensity of the external magnetic field Hex. Since the MI element is sensitive to an external magnetic field in the longitudinal direction of the element, the sensitivity for detecting a magnetic field does not deteriorate when the length of the sensor head is 1 mm or less. The MI element enables fabrication of a very weak magnetic field sensor having a high resolution of 10.sup.-5 Oe or more and excitation at several MHz or more, hence a high-frequency magnetic field of 200 MHz to 300 MHz can be used as a carrier for frequency modulation, and thus the cutoff frequency of the magnetic field sensor can be easily set to 10 MHz or more. Accordingly, the MI element is expected to be used in novel ultra-compact magnetic heads and sensors for very weak magnetic fields.
Known materials having MI effects include, for example, (1) amorphous ribbons of Fe--Si--B type alloys, e.g. Fe.sub.78 Si.sub.9 B.sub.13, and (2) amorphous wires of Fe--Co--Si--B system alloys, e.g. (Fe.sub.6 Co.sub.94).sub.72.5 Si.sub.12.5 B.sub.15 (Kaneo Mouri et al., "Magneto-Impedance (MI) Elements", Papers of Technical Meeting on Magnetics, MAG-94 (1994), Vol. 1, No. 75-84, pp. 27-36, IEE JAPAN).
The Fe--Si--B system and Fe--Co--Si--B system alloys have problems when they are used as MI elements. As shown in FIG. 6, when an output voltage Emi (mV) to a positive or negative magnetic field is measured, the Fe--Si--B system alloy i has low sensitivity for detecting the magnetic field, and thus a high amplitude of about 100 times is required. The element, therefore, cannot be used as a magnetic field sensor with a high sensitivity because of noise generation. On the other hand, although the Fe--Co--Si--B system alloy ii has a sufficiently high sensitivity, as shown in FIG. 6, it has a steep increase in the sensitivity within a range from -2 Oe to +2 Oe. As a result, it cannot be used as a sensing element for a very weak magnetic field due to non-quantitative measurement within the range. Although it can be used in magnetic field regions of 2 Oe or more as the absolute value, a coil must be provided to apply a considerable amount of current which is required for such a large bias magnetic field.
Recently, further miniaturization and further improvement in recording density have been required in magnetic recording units, such as hard disk drives as external memory units, digital audio tape recorders, and digital video tape recorders. Development of high performance magnetic heads is essential for such requirements, and magnetic reproduction heads using magnetoresistive elements (hereinafter referred to as MR elements) have been developed.
Since a magnetic head having a MR element does not have a dependence of a relative velocity to the recording medium, it is suitable for reading recorded signals at a low relative velocity. It has a low sensitivity to output signals because of a low change rate in response to a change in the recorded magnetization on the recording medium. Accordingly, it will be difficult to satisfy future demands for high-density recording.
Under the above-mentioned circumstances, MI elements have recently attracted attention. As described above, conventional MR elements have a magneto-detective sensitivity of about 0.1 Oe, whereas the MI elements having magneto-impedance effects can detect a magnetization of 10.sup.-5 Oe and are expected to be applied to high-sensitivity magnetic heads.
A typical conventional magnetic sensor of a magnetic head using the MI element will now be described with reference to the drawings. In FIGS. 28A and 28B, a magnetic head 201 has a pair of cores 202a and 202b composed of ferrite as a ferromagnetic oxide, and a MI element 205 as a magnetic material which is bonded to the cores 202a and 202b with a bonding glass 203 interposed therebetween. The MI element 205 is magnetically coupled with the cores 202a and 202b. That is, the ends 205a and 205b in the longitudinal direction of the MI element 205 are bonded to the magnetic circuit connecting faces 203a and 203b of the cores 202a and 202b, respectively. An insulating layer is formed on the magnetic circuit connecting faces 203a and 203b. The cores 202a and 202b and the MI element 205 thereby form a closed magnetic circuit.
The bonding glass 203 is composed of a nonmagnetic material, prevents direct magnetic coupling between the paired cores 202a and 202b, and is bonded to the lower faces of the cores 202a and 202b. A magnetic gap G is provided between the cores 202a and 202b. A regulating groove 204 is provided on the magnetic gap G for regulating the track width of the magnetic gap G, and filled with glass which is a nonmagnetic material. Conductive films composed of Cu, Au, or the like is deposited to form terminals 206a and 206b on the two ends of the MI element 205 in the longitudinal direction. The terminals 206a and 206b are each connected to a lead 207 for extracting output signals and a lead (not shown in the drawings) for applying an alternating current.
The magnetic head 201 operates as follows. An external magnetic field by the recorded magnetization on a recording medium not shown in the drawing invades the cores 202a and 202b through the magnetic gap G and is applied to the MI element 205. An alternating current having a MHz band has been previously applied to the MI element 205 to generate a voltage between both ends of the MI element 205 by the impedance inherent in the MI element. The amplitude of the voltage varies within a range of several tens percent in response to the intensity of the external magnetic field and is extracted as output signals through the lead 207.
The magnetic head 201 using the MI element 205 has a significantly high change in the extracted voltage for a very weak external magnetic field of several gausses which is applied to the MI element 205 from the recording medium, hence the magnetic head 201 can have high sensitivity. Further, such high sensitivity permits reduction in the effective cross-sectional area of the magnetic flux in the magnetic circuit, and thus reduction in the size of the cores 202a and 202b, resulting in miniaturization of the magnetic head 201.
Conventional materials used for MI elements are amorphous ribbons composed of Fe--Si--B system alloys, e.g. Fe.sub.78 Si.sub.9 B.sub.13 and amorphous wires composed of Fe--Co--Si--B system alloys, e.g. (Fe.sub.6 Co.sub.94).sub.72.5 Si.sub.12.5 B.sub.15. A magnetic head 201 using a MI element 205 composed of the Fe.sub.78 Si.sub.9 B.sub.13 alloy, however, produce a low output voltage from the MI element 205 for the applied external magnetic field. Thus, the output signals must be amplified by about 100 times. The element, therefore, cannot produce high quality output signals because of noise generation during the amplification.
On the other hand, a magnetic head 201 using a MI element 205 composed of the (Fe.sub.6 Co.sub.94).sub.72.5 Si.sub.12.5 B.sub.15 alloy produces a high voltage from the MI element 205 for the applied external magnetic field, resulting in a low amplification of the output signals. The output voltage, however, steeply and nonquantitatively varies within the very weak external magnetic field range from -2 Oe to +2 Oe. As a result, the MR element cannot be used as a magnetic field detecting element of the magnetic head. Although it can be used in a magnetic field region of 2 Oe or more as the absolute value, a coil must be provided to apply a considerable amount of current which is required for such a large bias magnetic field. When the bias magnetic field is applied from a permanent magnet having a magnetization of about 2 Oe, a complicated configuration of the magnetic head 201 is unavoidable.
In the Fe.sub.78 Si.sub.9 B.sub.13 and (Fe.sub.6 Co.sub.94).sub.72.5 Si.sub.12.5 B.sub.15 alloys, since a temperature region .DELTA.T.sub.x of the supercooling liquid is narrow, these must be quenched at a cooling rate of 10.sup.5.degree. C./second by a single roll process to form amorphous alloys. Thus, only a thin ribbon having a thickness of 50 .mu.m or less is obtainable. Fine working is necessary if these alloys are used for magnetic heads, resulting in increased production costs of the magnetic heads.
The present inventors have developed various types of alloys. One of them is a glassy alloy. Some multi-element alloys are known as glassy alloys having a wide temperature region in the supercooling liquid before crystallization. It is also known that glassy alloys can be obtained as a bulk having a thickness which is significantly larger than that of a thin ribbon of the amorphous alloy produced by a known liquid quenching process. Known glassy alloys have the following compositions, for example, Ln--Al--TM, Mg--Ln--TM, Zr--Al--TM, Hf--Al--Tm, and Ti--Zr--Be--TM, wherein Ln represents a rare earth element and TM represents a transfer metal.
Although these glassy alloys show a supercooling liquid state, the temperature region .DELTA.T.sub.x, that is, the difference (T.sub.x -T.sub.g) between the crystallization temperature (T.sub.x) and the glass transition temperature (T.sub.g) is small. Thus, these alloys have poor formability of glassy alloys which is insufficient for practical use. An alloy having a wide temperature region of the supercooling liquid state and enabling the formation of a glassy alloy by cooling metallurgically attracts considerable attention, since it can overcome the restriction regarding the thickness in conventional amorphous alloy ribbons. An alloy having a large difference .DELTA.T.sub.x enables a wide variety of deposition conditions in alloy thin film production processes, such as sputtering, and thus has industrial advantages. The alloy must, however, have ferromagnetic characteristics at room temperature before industrial use.
No glassy alloys having ferromagnetic characteristics at room temperature, however, have been known, hence their industrial application as magnetic materials has been limited. Accordingly, development of glassy alloys having ferromagnetic characteristics at room temperature and capable of forming a thick bulk has been in progress.
Azimuth sensors can measure the direction of the magnetic flux of an external magnetic field such as geomagnetism, and have been widely used as sensors for vehicle compasses and navigation systems which detect the location of vehicles.
Among the azimuth sensors, since a flux gate sensor shows excellent stability according to its operational principle and a high sensitivity of 10.sup.-7 to 10.sup.-6 G, it has been widely used. The flux gate sensor includes a cyclic magnetic core, an exciting coil coiled around the magnetic core for applying a magnetic field, and a sensing coil for detecting the magnetic flux density of the magnetic core. Thus, it has a bulky shape and is miniaturized with great difficulty.
Another azimuth sensor uses two MR elements. These MR elements are arranged in a common plane so that paths of the currents applied to these MR elements are perpendicular to each other and connected to a bridge to detect the direction of the magnetic flux of an external magnetic field. The azimuth sensor has a simplified shape and will be easily miniaturized.
An azimuth sensor using conventional MR elements, however, has a small rate of change in inherent resistance of 3% to 6% to the intensity of the external magnetic field. Such a nonsensitive change is unsuitable for accurate measurement of a magnetic flux of an external magnetic field such as geomagnetism and thus an azimuth sensor.
As a result of the trend towards high definition of CAD image information, the pitch of shadow mask holes in a display having a CRT tube (hereinafter referred to as a CRT display) has become finer. For example, a CRT display having a screen size of 14 inches has a pitch of 0.28 mm/mask. In such a high definition screen, electron beams in the CRT tube deviate from the objective lines by the effect of an external magnetic field such as geomagnetism, resulting in deterioration of image quality, e.g. distorted images, and uneven colors. Current CRT displays, therefore, have autocancelers for canceling the effect of the geomagnetism. The autocanceler has a canceling coil for applying a magnetic field having the reverse vector to the magnetic field of the geomagnetism, that is, a canceling magnetic field to the CRT tube, and a controller for controlling the vector of the canceling magnetic field.
A typical conventional controller for the autocanceler has a flux gate magnetic sensor having excellent stability according to its operational principle and a high sensitivity of 10.sup.-7 to 10.sup.-6 G. The flux gate sensor includes a cyclic magnetic core, an exciting coil coiled around the magnetic core for applying a magnetic field, and a sensing coil for detecting the magnetic flux density of the magnetic core. Thus, it has a bulky shape and is miniaturized with great difficulty.
Another magnetic sensor for the autocanceler uses two MR elements. These MR elements are arranged in a common plane so that paths of the currents applied to these MR elements are perpendicular to each other and connected to a bridge to detect the direction of the magnetic flux of an external magnetic field. The autocanceler has a simplified shape and will be easily miniaturized.
A magnetic sensor using conventional MR elements, however, has a small rate of change in inherent resistance of 3% to 6% to the intensity of the external magnetic field. Such a nonsensitive change is unsuitable for accurate measurement of a magnetic flux of an external magnetic field such as geomagnetism. Thus, the vector of the canceling magnetic field for normally operating the autocanceler cannot be optimized.